1. Field of the Invention
The present invention relates to an actuator having a movable plate allowed to move, and more particularly to an actuator that displaces a movable plate in directions vertical to a flat surface of the movable plate.
2. Description of the Related Art
Jpn. Pat. Appln. KOKAI Publication No. 2001-42233 discloses an actuator, which displaces a movable plate in directions vertical to a flat surface of the movable plate by using electrostatic driving. A structure of this actuator will now be described hereinafter with reference to FIGS. 17 and 18.
As shown in FIGS. 17 and 18, an actuator according to a prior art has a semiconductor silicon substrate 501, a movable electrode plate 502 supported by the semiconductor silicon substrate 501, an upper electrode 503 supported by the semiconductor silicon substrate 501 and positioned above the movable electrode plate 502, and a micro mirror 504 supported by the movable electrode plate 502.
The semiconductor silicon substrate 501 is formed of an n type semiconductor silicon, and constitutes a lower electrode. The semiconductor silicon substrate 501 has a concave portion 511 at its central portion, and a rim of the concave portion 511 constitutes an electrode support frame 512, which supports the movable electrode plate 502 and the upper electrode 503. The concave portion 511 provides a space into which the movable electrode plate 502 moves by suction.
The movable electrode plate 502 is formed by using polysilicon as a material. The movable electrode plate 502 includes a flexure portion 521 and anchor portions 522, both of which are integrally formed, and is fixed to the semiconductor silicon substrate 501 through the anchor portions 522.
The upper electrode 503 is also formed by using polysilicon as a material like the movable electrode plate 502. The upper electrode 503 has upper electrode anchor portions 531 and raised portions 532, both of which are integrally formed, and is fixed to the semiconductor silicon substrate 501 through the upper electrode anchor portions 531. Further, the upper electrode 503 has, at its central portion, a through hole 533, through which the micro mirror 504 is passed.
The micro mirror 504 is erectly fixed on an upper surface of the movable electrode plate 502. A reflection surface of the micro mirror 504 is inclined at, e.g., 45° with respect to an advancing direction of light beams.
In this actuator, a voltage is selectively applied between the movable electrode plate 502 and the upper electrode 503 or between the movable electrode plate 502 and the lower electrode, i.e., the semiconductor silicon substrate 501. The movable electrode plate 502 moves up and down in accordance with such a selective voltage application. As a result, when the micro mirror 504 is inserted into or removed from a light path, a light switch is turned on or off.
Since an electrostatic drive mode is adopted for the above-described electrostatic drive type actuator, a relationship between an applied drive voltage and a displacement quantity of a movable plate (movable electrode plate) has non-linearity.
Furthermore, in the electrostatic drive mode, a sticking state may occur between the movable plate and the fixed electrode portion due to pull-in at the time of a large displacement. The movable plate and the fixed electrode portion must be separated from each other by a distance which is approximately three times of the maximum displacement quantity of the movable plate in a stationary state in order to avoid occurrence of such a sticking state.